As a result of, inter alia, the increased use of multimedia computers, the demand for higher density storage in optical media is increasing. The capacity of optical media (i.e., optical disk and/or the like), which is typically based on the density of the information on the optical media, has substantially increased in recent years and exponential growth in the capacity of optical media is expected in the next few years. As an example of the density increase, the currently marketed 4X generation of magneto-optical media commonly has a capacity of about 2.6 GB and the 8X generation currently being developed would have a capacity of about 5.2 GB. The following discussion is generally related to magneto-optical disk systems; however, the basic concepts relating to read out signal and crosstalk etc. are generally applicable to other types of optical disk systems, though specific reading, erasing and recording mechanisms may be different.
When increasing the capacity of an optical disk, the separation of the spiral tracks (each track is commonly comprised of a groove between two lands) typically formed on the surface of the optical disk is substantially reduced such that the individual track/land are typically less than 1 .mu.m apart from each other. Numerous marks (also known as domains), the sizes of which are typically determined by the writing laser spot size (a "spot" is a common term for the focused laser beam often limited by diffraction) and the length of a binary representation of a data field, are commonly recorded in the grooves between the track lands (see FIG. 3). These marks may also be written on the lands or on both land and groove. Due to the decreased distance between adjacent tracks on the high density optical disk, the formation and detection of a mark in a groove between two adjacent track lands often becomes increasingly difficult. Similar difficulty typically exists for writing/reading marks on the lands.
To fit within a track, a sufficiently small optical beam spot is typically required. Laser wavelength and the numerical aperture of the lens used for the writing device typically determine the beam spot size, and consequently, the size of each mark. Shorter wavelength and higher numerical aperture provide smaller spot size and smaller marks result in increased storage density. Thus, a high power semi-conductor red laser (typical wavelength 685 nm) is often utilized when writing the magnetic code onto the optical disk. However, typically lasers with wavelengths between 650 and 685 nm are currently utilized by the optical disk storage industry, though laboratory prototypes of shorter wavelength lasers are emerging. Moreover, the numerical aperture is mathematically restricted to be less than 1.0 in common implementations. Thus, a further substantial reduction in the size of the optical beam spot by a shorter wavelength or larger numerical aperture currently presents practical problems.
Because of the limitations in reducing the size of the focused optical beam, the beam spot size is often larger than the width of a single track in a high density optical disk and, often the laser energy extends over into the adjacent track, thereby resulting in a problem known as adjacent track crosstalk (ATC). ATC becomes a more pronounced problem when writing low frequency data onto a high-density optical disk (i.e., 8X generation and denser) because the low frequency data typically forms a larger mark contributing to more signal and more crosstalk. Because the beam spot is often larger than an individual track in a high density disk and often extends into the adjacent track, the data contained within the larger mark in the adjacent track is partially read when the reading process occurs on the main track, thereby resulting in crosstalk from the adjacent tracks (see FIG. 3).
Specifically, when reading from a disk, the laser beam commonly analyzes each mark within the track. When ATC exists, the data contained within the large masks in neighboring tracks is partially sensed by the read focused spot when the reading process occurs on the adjacent track. The amount of crosstalk that is coupled to the read focus spot is typically proportional to the size of the mark and spacing between marks. For example, and as shown in FIG. 3, when reading a 2T pattern, which is the smaller mark size pattern, the read focused spot may sense an 8T pattern (large marks) located in an adjacent track. If the amount of the adjacent track signal pattern which is sensed by the read focused spot is greater than about 10% of the 2T pattern signal amplitude, excessive jitter typically occurs, thereby decreasing the reliability in the read channel.
During a typical reading process from a magneto-optical disk, the laser beam, after having its polarization state altered by the magnetic material of an individual mark on the optical disk, is usually analyzed by a detector and associated electronic circuits. The intensity distribution across the focused spot usually follows what is called the Airy pattern. Upon reflection from the disk, the high frequency small mark pattern typically diffracts light toward the side perimeter of the collecting optics aperture, while the low frequency large mark pattern diffracts light towards the center of the aperture (see FIG. 4). Noise, however, is often distributed randomly throughout the readout aperture.
A known method for substantially reducing ATC is to selectively suppress the low frequency content from the adjacent track in the readout beam while enhancing the high frequency response because, as discussed, the low frequency large mark pattern from the adjacent tracks give unwanted overlap and the high frequency pattern emanates from the high frequency marks which generate substantially less ATC. To selectively reduce the low frequency content from the adjacent track, a technique known as "apodization" is employed whereby a narrow shading band is placed in front of the detector or the readout beam path. The shading band, because of its carefully selected central location, often substantially blocks out the crosstalk section of the beam in the readout path. Moreover, by selectively blocking out the unwanted portion of the beam in the readout aperture, a large portion of the noise contained within that section of the spot is also blocked out, thereby increasing the signal to noise ratio. Consequently, this technique can provide up to about 10 dB improvement of ATC. The shading band often comprises a thin rectangular strip of any suitable material or a metal wire of chosen diameter, typically covering about 20 to 30% of the beam size. The aforementioned apodization technique is described in more detail in "High-frequency enhancement of magneto-optic data storage signals by optical and electronic filtering" by Edwin P. Walker and Tom D. Milster, Optics Letters, Volume 20, No. 17 (Sep. 1, 1995), pp. 1815-1817, "Crosstalk reduced by new types of optical filtering" by Takeshi Shimano, et al., presented at the 1997 Optical Data Storage Topical Meeting on Apr. 7-9, 1997 and "Crosstalk-Suppressed Read-out System Using Shading Band" by Hisanobu Dobashi, Takaya Tanabe and Manabu Yamamoto, Journal of Applied Physics, Volume 36 (January 1997), pp. 450-455, which are herein incorporated by reference.
Apodization is typically a powerful method for selectively suppressing the low frequency portion of the diffraction pattern and thereby decreasing the ATC when reading high density optical disks. However, an optical disk reader which is constructed with a shading band in front of the detector or in the readout optical path is typically only optimally effective when reading optical disks of sufficiently high density (i.e., 8X generation and denser) which display ATC problems. In other words, the optical disk drives with such apodization device for the 8X generation optical disks are typically not effectively backward compatible for the prior generation lower density disks which commonly do not experience substantial ATC. Therefore, when an earlier generation, lower capacity, optical disk is inserted into a high density optical disk drive, the shading band, which is a permanent part of the high density optical disk drive and limited to a permanent configuration, will often still substantially reduce or eliminate signal content in the diffraction pattern in the masked portion of the readout beam even though the masked portion does not necessarily comprise the unwanted ATC. Thus, when a lower density, earlier generation, optical disk is inserted into an optical disk drive which is configured with a shading band, a significant reduction in valuable signal information can occur without any substantial benefit.